The present invention relates to a method of estimating the thermal stress of a heat-resistant member, part or the whole of which is to be heated to such a high temperature as 500.degree. C. or higher.
Heat-insulating members repeatedly placed under heated and cooled conditions, for instance, exhaust gas equipment such as exhaust manifolds used in internal combustion engines of automobiles, etc., are required to be designed such that they undergo a minimum thermal strain, to prevent their breakage such as by cracking during operation. For instance, in the case of exhaust manifolds of automobiles, they are subjected to increasingly higher temperatures and thermal loads, because engines having increasingly higher performance have been developed. Therefore, the exhaust manifolds should be designed such that thermal cracks are prevented from being generating due to thermal strains.
Conventionally, the measurement of a thermal strain distribution of a manifold, etc. is conducted by using high-temperature strain gauges as disclosed in Japanese Patent Publication No. 51-42947 when the temperature of measurement is 200.degree. C. or higher. However, the high-temperature strain gauges have high stiffnesses and suffer from thermal strain because they are attached to the measurement sites of a manifold by welding. Accordingly, in portions having small radii of curvature, reliable data cannot be obtained due to the gauges peeling off from the measurement sites. Further, even with the above strain gauges, the linearity of measurement data will be lost at a temperature higher than 500.degree. C. Therefore, the strain distribution on the overall manifold surface at a high temperature was conventionally presumed from data at 500.degree. C. or lower.
In addition, in the above method, test samples are produced by casting and subjected to a bench test, and the modification of cast product designs is conducted based on the data obtained with respect to cast samples. Accordingly, such a method requires a long period of time for determining the best design of a heat-resistant member such as an exhaust manifold, resulting in high development costs.